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      <div class="post-title">Optional类</div>
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          2021-09-24 10:21:11
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        <figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">final</span> <span class="keyword">class</span> <span class="title class_">Optional</span>&lt;T&gt; <span class="keyword">extends</span> <span class="title class_">Object</span></span><br></pre></td></tr></table></figure>

<p>可能包含或不包含非空值的容器对象。 如果一个值存在， <code>isPresent()</code>将返回<code>true</code>并且<code>get()</code>将返回值。</p>
<p>提供依赖于存在或不存在包含值的其他方法，例如<a target="_blank" rel="noopener" href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Optional.html#orElse-T-"><code>orElse()</code></a> （如果值不存在则返回默认值）和<a target="_blank" rel="noopener" href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Optional.html#ifPresent-java.util.function.Consumer-"><code>ifPresent()</code></a> （如果值存在<a target="_blank" rel="noopener" href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/util/Optional.html#ifPresent-java.util.function.Consumer-">则</a>执行代码块）。</p>
<p>这是一个<a target="_blank" rel="noopener" href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/doc-files/ValueBased.html">value-based</a>类; 使用身份敏感的操作（包括引用相等（<code>==</code>），标识哈希码，或同步） <code>Optional</code>可具有不可预测的结果，应当避免。</p>
<table>
<thead>
<tr>
<th align="left">Modifier and Type</th>
<th align="left">Method and Description</th>
</tr>
</thead>
<tbody><tr>
<td align="left"><code>static &lt;T&gt; Optional&lt;T&gt;</code></td>
<td align="left"><code>empty()</code><br/>返回一个空的 <code>Optional</code>实例。</td>
</tr>
<tr>
<td align="left"><code>boolean</code></td>
<td align="left"><code>equals(Object obj)</code><br/>指示某个其他对象是否等于此可选项。</td>
</tr>
<tr>
<td align="left"><code>Optional&lt;T&gt;</code></td>
<td align="left"><code>filter(Predicate&lt;? super T&gt; predicate)</code><br>如果一个值存在，并且该值给定的谓词相匹配时，返回一个 <code>Optional</code>描述的值，否则返回一个空的 <code>Optional</code> 。</td>
</tr>
<tr>
<td align="left"><code>&lt;U&gt; Optional&lt;U&gt;</code></td>
<td align="left"><code>flatMap(Function&lt;? super T,Optional&lt;U&gt;&gt; mapper)</code><br/>如果一个值存在，应用提供的 <code>Optional</code>映射函数给它，返回该结果，否则返回一个空的 <code>Optional</code> 。</td>
</tr>
<tr>
<td align="left"><code>T</code></td>
<td align="left"><code>get()</code><br/>如果 <code>Optional</code>中有一个值，返回值，否则抛出 <code>NoSuchElementException</code> 。</td>
</tr>
<tr>
<td align="left"><code>int</code></td>
<td align="left"><code>hashCode()</code><br/>返回当前值的哈希码值（如果有的话），如果没有值，则返回0（零）。</td>
</tr>
<tr>
<td align="left"><code>void</code></td>
<td align="left"><code>ifPresent(Consumer&lt;? super T&gt; consumer)</code><br/>如果存在值，则使用该值调用指定的消费者，否则不执行任何操作。</td>
</tr>
<tr>
<td align="left"><code>boolean</code></td>
<td align="left"><code>isPresent()</code><br/>返回 <code>true</code>如果存在值，否则为 <code>false</code> 。</td>
</tr>
<tr>
<td align="left"><code>&lt;U&gt; Optional&lt;U&gt;</code></td>
<td align="left"><code>map(Function&lt;? super T,? extends U&gt; mapper)</code><br/>如果存在一个值，则应用提供的映射函数，如果结果不为空，则返回一个 <code>Optional</code>结果的 <code>Optional</code> 。</td>
</tr>
<tr>
<td align="left"><code>static &lt;T&gt; Optional&lt;T&gt;</code></td>
<td align="left"><code>of(T value)</code><br/>返回具有 <code>Optional</code>的当前非空值的Optional。</td>
</tr>
<tr>
<td align="left"><code>static &lt;T&gt; Optional&lt;T&gt;</code></td>
<td align="left"><code>ofNullable(T value)</code><br/>返回一个 <code>Optional</code>指定值的Optional，如果非空，则返回一个空的 <code>Optional</code> 。</td>
</tr>
<tr>
<td align="left"><code>T</code></td>
<td align="left"><code>orElse(T other)</code><br/>返回值如果存在，否则返回 <code>other</code> 。</td>
</tr>
<tr>
<td align="left"><code>T</code></td>
<td align="left"><code>orElseGet(Supplier&lt;? extends T&gt; other)</code><br/>返回值（如果存在），否则调用 <code>other</code>并返回该调用的结果。</td>
</tr>
<tr>
<td align="left"><code>&lt;X extends Throwable&gt;T</code></td>
<td align="left"><code>orElseThrow(Supplier&lt;? extends X&gt; exceptionSupplier)</code><br/>返回包含的值（如果存在），否则抛出由提供的供应商创建的异常。</td>
</tr>
<tr>
<td align="left"><code>String</code></td>
<td align="left"><code>toString()</code><br/>返回此可选的非空字符串表示，适用于调试。</td>
</tr>
</tbody></table>
<ul>
<li><h3 id="从-java-lang-Object类继承的方法"><a href="#从-java-lang-Object类继承的方法" class="headerlink" title="从 java.lang.Object类继承的方法"></a>从 java.lang.<a target="_blank" rel="noopener" href="https://www.matools.com/file/manual/jdk_api_1.8_google/java/lang/Object.html">Object</a>类继承的方法</h3><figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">clone, finalize, getClass, notify, notifyAll, wait, wait, wait</span><br></pre></td></tr></table></figure></li>
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